Actylcholinesterase inhibitors or cholinesterase inhibitors reduce the degradation of acetylcholine thereby enhancing cholinergic transmission. A number of cholinesterase inhibitors, including tacrine (COGNEX®), galantamine (RAZADYNE®), rivastigmine (EXELON®), and donepezil (ARICEPT®) are approved for administration to Alzheimer's disease (AD) patients. Other compounds including physostigmine are under investigation as potential therapy for Alzheimer's disease. These compounds offer relatively similar mean gains in cognitive abilities to patients with AD in controlled clinical trials, and they have similar side effects—nausea, vomiting or diarrhea.